Yes, if your new OS has easy & flexible installation procedure like Linux/BSD where you can specify on which partition you want to install, so that you dont overwrite existing OS partition. You will also need to check the details of the bootloader of your new OS, whether like Linux/BSD it allows to boot all the installed OSes.
1. Is this backwards installable? Let me clarify that. I noticed that if you already have another OS (I think Windows works) installed on your computer, you can run Ubuntu along the other OS. My question is, can you do that backwards? Meaning, can you install Ubuntu, then once I get enough money for an OS, can I install it alongside Ubuntu?
I dont know from where you got this misconception. Linux based OSes are free & opensource. They give the user freedom to do anything & everything with the OS, unlike a propreitary OS where everything is restricted. Any free & opensource software or game will run on Linux, as you can always compile it even if they dont provide the binary for your flavor of Linux. Also, there are many propreitary Linux software & games, but they only provide binaries for well-known Linux OSes. Searching on www for your specific need of software or games would be a good idea.
2. Is it really true that there isn't that much stuff you can do on Linux? I know that there aren't that many things that are downloadable for Linux. But I would like to know how much there really is that you can't do. Like, game and software wise.
Yes, depending on the size. Just burn it, no specific software needed.
3. Can I burn to a CD instead of a DVD? And if so, is there a specific software I should use? If you could point me toward a very in depth tutorial, that would be great.